Bombesin-related peptides ([gastrin-releasing peptide [GRP], neuromedin B) interact with two distinct receptors (GRP-R, NMB-R) to mediate a number of effects in the gastrointestinal tract (GI), central nervous sytem (CNS) and on growth of normal and neoplastic tissues. Furthermore, two related receptors, a mammalian orphan receptor (BRS-3), having 60% homology to GRP-R and a novel receptor in amphibians, BB-4-R has been described recently. The aims of this project are to understand the pharmacology, molecular pharmacology, and cell biology of these receptors as well as to develop specific agonists and antagonists that can be used to determine their physiological roles. Investigations being performed include expression of these receptors in stable cell lines that resemble native receptors in their cell biology and pharmacology; investigations using site-directed mutagenesis and receptor chimeras to define receptor structural determinants of ligand selectivity and specificity for agonists and antagonists, pharmacological studies of BN-related peptides to identify selective agonists/antagonists and studies of native cells and transfected cells to define the transduction cascades of these receptors. Using site directed mutagenesis and a chimeric receptor approach, we have examined the molecular basis of the selectivity of agonist GRP (2), high GRPR peptide antagonists (4) and the NMB receptor antagonist, PD168368 (3). These results demonstrate the critical amino acids in the 3rd extracellular domain (EC-3) are responsible for GRP selectivity, three amino acids in EC4 of the GRPR responsible for the antagonist's high GRPR selectivity and the amino acids in upper 5th transmembrane region of the NMBR responsible for PD168368's selectivity. Molecular modeling and site-directed mutagenesis studies showed this selectivity was primarily mediated by hydrogen bonding and pi-cation interactions between these ligands and the receptor. Using receptor modeling and mutagenesis studies we have recently designed a selective agonist for the BRS-3 receptor (1). Prior to the BRS-3 study, no selective ligands existed for the BRS-3 receptor restricting the ability to investigate its physiological roles. Using conformationally restricted analogues of [B-Ala11] bombesin and modeling we identified an amino-3 phenyl propionic acid analogue as the first selective BRS-3 agonist.